Why is a 50 percent recovery of single-crossover products the upper limit, even when crossing over always occurs between two linked genes?

Because of homologous recombination

Explanation:

When genes are establish on different DNAs or far apart on the same chromosome, they are classified self-sufficiently and are said to be unlinked. When genes are very close together on the same chromosome, they are said to be linked. That means that alleles, or genetic versions, that are already together on a chromosome will be inherited as a unit more often than not. We can see if two genes are linked, and how closely, by using data from genetic crosses to calculate the frequency of recombination. Using the technique of discovery recombination happenings for numerous gene pairs, we can make link maps that show the order and relative distances of the genes on the chromosome.

When the genes are on the same chromosome but far apart, they are classified independently due to crossing (homologous recombination). This is a procedure that happens at the start of meiosis, in which homologous DNAs randomly exchange matching fragments. Crossing be able to connection new alleles in combination on the same chromosome, causing them to enter the same gamete. When the genes are far apart, the crossing occurs with sufficient frequency for all types of gametes to occur with 25% percentage frequency. When the genes are very close together on the same chromosome, the crossing still occurs, but the result (in terms of the types of gametes produced) is different. Instead of being classified independently, genes tend to "stay together" during meiosis. That is, alleles of genes that are already together on a chromosome will tend to pass as a unit to gametes. In this case, the genes are linked